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Nuns register for their Pfizer/BioNTech vaccine
Nuns register at an inoculation centre in Tel Aviv. Israel has vaccinated a large proportion of its older population. Photograph: Jack Guez/AFP/Getty
Nuns register at an inoculation centre in Tel Aviv. Israel has vaccinated a large proportion of its older population. Photograph: Jack Guez/AFP/Getty

Pfizer/BioNTech vaccine reducing viral load, data from Israel suggests

This article is more than 3 years old

Initial study brings hope vaccine will reduce Covid transmission

Data from researchers in Israel, which has inoculated swathes of its population, suggests the Pfizer/BioNTech vaccine is reducing viral load, a key signal that the intervention could diminish the spread of Covid-19.

Evidence that the coronavirus vaccines being deployed globally are dramatically effective in reducing severe disease and death in symptomatic Covid-19 is abundant. But a big question remains unanswered: can they thwart transmission, in other words stop people from passing on the virus?

Preventing the spread of infection is key to reducing the risk of more variants emerging and to achieving herd immunity, scientists say. People with higher viral load tend to be more infectious and are more likely to suffer from severe disease.

To evaluate the impact of the vaccine on transmission, researchers compared data from people over 60 years old and those aged 40 to 60, evaluating data from 16,297 people who had tested positive for coronavirus between 1 December and 30 January.

Israel’s vaccine programme began on 20 December. By the time of analysis, more than 75% of those in the older group were likely to have received their first dose, as had about 25% of the younger group.

Quick Guide

Vaccines: how effective is each one?

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Pfizer/BioNTech

Country US/Germany

Efficacy 95% a week after the second shot. Pfizer says it is only 52% after the first dose but the UK’s Joint Committee on Vaccination and Immunisation (JCVI) says this may rise to 90% after 21 days.

Doses Clinical trials involved two doses 21 days apart. The UK is stretching this to 12 weeks.


Oxford/AstraZeneca

Country UK

Efficacy 70.4% 14 days after receiving the second dose. May have up to 90% efficacy when given as a half dose followed by a full dose. No severe disease or hospitalisations in anyone who received the vaccine. 

There have been concerns it is less effective against the South African variant of the coronavirus, and that a rare type of blood clot can be a side effect.

Doses Two, four to 12 weeks apart


Moderna

Country US

Efficacy Phase 3 trial results suggest 94.1%.

Doses Two, 28 days apart


Novavax

Country US

Efficacy Phase 3 trials suggest 89.3%.

Doses Two


Janssen (part of Johnson & Johnson)

Country US

Efficacy 72% in preventing mild to moderate cases in US trials but 66% efficacy observed in international trials. 85% efficacy against severe illness, and 100% protection against hospitalisation and death.

Doses: One, making it unique among Covid vaccines with phase 3 results so far


Sinovac

Country China

Efficacy Unclear. Researchers in Brazil have found the efficacy of the Chinese vaccine to be as low as 50.4%, whereas Turkish officials reported interim data from trials showing efficacy was 91%.

Photograph: Stéphane Mahé/X02520
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Although researchers did not know whether each person had been given their first vaccine dose, their hypothesis was that if the vaccine was reducing viral load, then evidence of that would begin to show up in late January but not before, because of the time required by the vaccine to stimulate the immune system.

As expected, in the last two weeks of January the researchers noted a statistically significant fall in the viral load for the individuals aged over 60, compared with the 40-to-60 group, said the study author, Yaniv Erlich, the chief scientific officer of MyHeritage, a company that runs a large coronavirus testing laboratory in Israel.

The researchers used available demographic data and vaccination rates to estimate the effect of the first dose on viral load reduction, calculating that the vaccine reduced the viral load by 1.6 to 20 times in individuals who tested positive for the virus.

Erlich cautioned that the paper, which has yet to be peer-reviewed or published in a medical journal, was just an initial study. While it was understood that a smaller viral load was better, it was not clear whether this reduction would be enough to block transmission. “My expectation is … that if you are positive following vaccination, you probably will transmit the disease to a smaller number of people, on average,” he said.

Stephen Griffin, associate professor at Leeds University’s school of medicine, said the data, though early, was positive but not definitive. Since the study looked at people who had tested positive for the virus, the results suggested that vaccinated people were less likely to pass infection on, potentially, by virtue of the fact they had a reduced virus load, he said.

It remained unclear whether the vaccine stopped people from getting infected altogether – however, this study was not designed to assess that, Griffin said.

Last week, Oxford/AstraZeneca unveiled a preliminary analysis from a small subset of patients that hinted that one dose of their vaccine could cut transmission of the virus by up to 67%, on the basis of a reduction in viral load.

More on this story

More on this story

  • Israeli real-world data on Pfizer vaccine shows high Covid protection

  • Coronavirus vaccine strategy needs rethink after resistant variants emerge, say scientists

  • Two new Covid vaccines have less efficacy against South African strain

  • Johnson & Johnson one-dose Covid vaccine shown to work

  • Novavax Covid vaccine shows nearly 90% efficacy in UK trial

  • Will Novavax and Johnson & Johnson Covid vaccines work against variants?

  • MHRA and vaccine makers in talks over new Covid variants

  • Moderna Covid vaccine approved for use in UK

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